Method and means for automatic gain control



` June 29, 1943. F.J.FALT1O 2,323,211

METHOD AND MEANS FOR AUTOMATIC GAIN CONTROL Filed May 23, 1941Sheets-Sheet 1 l -ikf www f I l /0 1 /4 V2 /2 [/3 v n June 29, 1943. F,J. FAL-rico 2,323,211. A METHOD AND MEANS FOR AUTOMATIC GAIN CONTROLFiled Mayas, 1941 2 sheets-sheet 2 Femm J. FALTNO f Tn/Patented June 29,1943 UNITED STATES PATENT OFFICE METHOD AND MEANS FOR AUTOMATIC GAINCONTROL ll Claims.

This invention relates to improvements in automatic gain controls forvacuum tube amplifier systems.

The use of automatic means for controlling the gain or volume of anamplifier is well known in the art but all systems with which I amfamiliar have had certain disadvantages chief of which has been theconsiderable time lag in the action involved in the use of reactivenetworks of various kinds.

It is, of course, extremely desirable to make the control not onlyautomatic but practically instantaneous in order to reduce the gain theinstant the signal builds up beyond a predetermined level and thusprevent overloading of the output circuit, distortion, and other badeffects. Particularly is this action desirable in amplifiers for hearingaid work in which the greatest possible gain must, be present but inwhich a loud signal, if not instantly suppressed, would cause extremedistortion and possibly reach the threshold of pain for the user.

It is the primary object of my invention, therefore, to provide a methodof. and means for, automatically and, to all useful purposesinstantaneously, controlling the gain or volume in an amplifier circuit.Another object is to provide a gain control of this kind which utilizescertain principles of the operation of the vacuum tubes of the amplifierand associated circuits to provide the desired control without affectingthe gain on weak s'gnals and without adding parts to the circuit. Afurther object is to provide an automatic gain control of extremesimplicity and which may be initially set up to limit the gain to anydesired degree.

These and other more detailed and specific bjects will be disclosed inthe course of the following specication, reference being had to theaccompanying drawings, in which- Fig. l is a wiring diagram of a threetube amplifier circuit embodying my invention.

Ffg. 2 is a graphic showing of the progress of a wave through the threetube amplifier of Fig. 1.

Fig. 3 is a diagrammatic showing of the output curve of an amplifierconnected in accordance with my invention, several possible outputlevels being indicated.

Figs. 4 through 7 are wiring diagrams of amplifiers embodying certainvariations or modifications of the gain control means of Fig. 1.

My invention as disclosed herein is particularly adapted for use inaudio frequency amplifiers employing screen grid pentode or tetrodetubes,

although it may, of course, be used in connection with amplifiershandling other frequencies, wherever feasible.

In such amplifiers, unless some form of gain or volume control is used,there is always the probability of overloading and distortion since thesignals handled may frequently exceed the capabilities of the tubes andnon-linear amplication will then result. Particularly is this true wherethe circuits and tubes are initially selected and set up to have highamplification for weak signal response. In hearing aid Work thisdistortion and overloading is especially noticeable due to the widerange of signal intensities handled and limitations as to operatingvoltages imposed by the necessary compactness and light weight of suchunits. The distortion is usually met with in the output stage butfrequently occurs also in the preceding or driving stage, and maycontain both even and odd harmonics. The even harmonic distortion may belargely eliminated by the use of push-pull output stages or other means,as frequently found in power or radio amplifiers, while the odd harmonicdistortion, usually of the third order, may be tolerated to no greatdisadvantage since it will not be evident to the usual ear.

From the foregoing it may be assumed therefore that in practically allampliers there is some distortion caused by overloading and I propose toemploy this factor to serve as a control for the circuit toautomatically reduce the gain and then cause the elimination of suchdistortion, at least to a large extent, in the output signal, Itherefore deliberately introduce distortion in one or more stages of theamplifier and later largely cancel out the distortion after it hasserved its useful purpose, my system being thus what might be termed adistortion-dedistortion type of gain control system.

To introduce such distortion I propose to operate one or more of thevacuum tubes at a point on its static or gain characteristic curve atwhich distortion and suppression of the signal will occur above acertain input signal, this being accomplished by setting up theoperating potentials and bias on the tube or tubes to the point whereboth plate and screen grid currents are affected by an increase insignal levelabove a predetermined maximum. In other words, the operatingpoint is such that, on increasing the control grid voltage (largesignal) the screen grid voltage does not rise in normal ratio to thecontrol grid voltage, or does not rise above a predetermined value.

The effect of distortion thus entered by a large signal is to shift thedirect current component of the signal in the output of the tube andthus cause a relatively great increase in the plate current drawn by thetube or tubes above the normal or idling value. I then cause thiscurrent to flow through a resistance common to both the plate and screengrid of one or more tubes and the iR drop thus secured is employed toautoa matically reduce or limit the screen grid voltage. l It ls wellknown in the art that the behavior of a screen grid amplifier tube issuch that large plate load resistances may be used since the spacecharge in the tube, and hence the gain, will be held up by the screengrid element. Thus in the usual amplifier with high values of loadresistance and with plate current variations having little, if any,effect on space charge conditions, maximum gain is secured. Theimportance of the operating potential on the screen grid element is thusevident since, as the instantaneous plate voltage drops on the positivehalf of each control grid voltage swing, the ratio of the screen gridvoltage relative to plate voltage must increase in order to hold upspace charge conditions and any deviation, particularly toward a lowervalue of screen grid voltage, will radically reduce the space charge andgain.

Briefly therefore my invention resides in the introduction of distortionin an amplifier to secure a shift in the direct current component of thesignal, the use of the plate current variation thus secured to cause avoltage drop to the screen grid element or elements in the amplifiertubes, and the later cancellation of the undesirable values of thedistortion to provide an output signal substantially a replica of theinput signal.

Referring now more particularly to the drawings, I show in Fig. 1thereof a circuit by which the aforesaid operations may be carried out.This circuit employs, in at least the preampliiying portions, threescreen grid pentodc or tetrode vacuum tubes V1, V2 and V3 which inaddition to the normal control grids, filaments, suppressor grids andanodes or plates, the screen grids designed at I0, il and I2 in therespective tubes, these elements service as aforesaid to maintain spacecharge condition during normal operation. The signal input, from amicrophone, or other detection means, is indicated at I to the controlgrid circuit of tube Vl while the output from tube V3 is indicated at O.This output may be connected to a translator of any kind but to greateradvantage, as will later appear, should be used to drive a furtheramplifying stage, or stages, of normal characteristics.

The amplifier circuit is of the resistance coupled type and has theusual control grid bias resistors I3 for each tube, interstage couplingcondensers I4 and plate load resistances I5, I6 and I'I for therespective tubes V1, V2 and V3. These plate load devices, of highresistance, are connected to the source of plate potential, or Bpositive, at I8, and interstage decoupling may be provided by resistorsI9 and by-pass condensers 20. The last tube V3 has its screen gridelement I2 connected directly to B positive I8 through dropping resistor2l and is by-passed by condenser 22, the voltage for this element beingthus conventionally secured.

In accordance with my invention however the screen grid element I of thefirst tube V1 is connected through dropping resistor 23 to the plate oftube V2 so that the load resistor I6 for this tube V2 is common to itsplate and t0 the out of phase with each other.

screen grid element ID. In similar manner the screen grid element II oftube V2 is connected to the plate of tube V3 through resistor 24, thusmaking the load resistor Ii common to the plate of V3 and screen grid ofV2. Both screen grid elements I0 and II are shown as by-passed to groundby condensers 25, the use and value of which will be later discussed.

Operating potentials are thus correctly supplied to all tubes V1, V2 andV3 and to a certain extent, or upon signals below a predeterminedmaximum intensity, the amplifier thus connected will operate in usualfashion.

Referring to Fig. 2 the exact operation will now be described. In thisillustration there is shown at A1, A2 and A3 the static or gaincharacteristic curves of the tubes V1, V2 and V3, respectively, asplotted against plate current and control grid voltage. These curveshave the substantially linear midportions and each bend at one end Btoward an upper saturation point and at the other end or foot C toward alower cut-off point.

The rst amplifier tube Vl is initially chosen and operating potentialsset so that it operates near the center of the linear portion of thecurve A1 at an operating point PI as indicated. An alternating currentinput signal or wave W is also shown and is plotted as of such amplitudeas to pass through tube V1 without distortion, receiving linearamplification thereby. When this wave W is superposed upon the curve A1about an axis at the operating point P1 then the resulting output waveW0, due to the slope of the curve A1, will be seen to be linearlyamplified by the gain of the tube. Operation thus far is normal.

The second tube V2 is however set, by preselection of operatingconditions, to operate at an operating point P2 between the center ofthe linear portion of curve A2 and the lower bend C thereof. Now as thesignal W0 is impressed upon this curve A2 about an axis coinciding withoperating point P2 the output wave from tube V2, shown at W2. is seen tobe suppressed upon one side X due to the overloading of the tube undersuch conditions. There results then a condition of distortion and ashift, as designated at S2, in the direct current component of thesignal, and the magnitude of this shift is determined not only by theamplitude of the signal but upon the position initially chosen foroperating point P2.

The third tube V3 is similarly preset to operate at a point P3 towardthe lower bend C of its curve A3 and as the wave W2 is then transferredto curve A3 about the axis the resulting output Wave W3 is seen to besimilarly nonlinearly amplified but suppressed upon the side Y oppositeto that of wave W2. This is, of course, due to the fact that the tubesV2 and V3 operate 180 degrees There again occurs a shift S3 in thedirect current component of the signal in the plate circuit of tube V3.

Referring now again to Fig. 1 the result of the direct current componentshift S2, through the plate resistor I6 to tube V2, is to increase thecurrent flowing through such resistor and accordingly cause aninstantaneous voltage drop therethrough by the iR. formula. Due to mynovel circuit, however, this voltage drop is caused to affect theoperating voltage on the screen grid element I0 of tube V1 instantlyreducing the voltage thereon and hence lowering space charge conditionsand gain in the first amplifier. In similar manner the current swing(upward) through resistor I1 in the plate circuit of tube V3, caused bythe direct current component shift S3, is caused to drop the voltageapplied to the screen grid element II of preceding tube V2 reducingspace charge and gain therein. There thus results a control of the gainthrough both tubes V1 and V2 directly responsive to the signal amplitudein tubes V2 and V3. Due to the out of phase operation of what might betermed the gain controlling tubes V2 and V3 the gain suppression orreduction takes place on both positive and negative halves of each cycleas should be clearly evident.

The effect of the screen grid by-pass condensers 25 is to by-pass secondharmonics or alternating current components of the signal and the valueof these condensers is somewhat critical since they have a tendency tohold up the screen grid voltage, causing some time lag in the automaticgain control operation. By proper choice of the capacity used this timelag may however, be reduced to a negligible value for audio frequencywork.

Preferably the output wave W3 from tube V3 is fed to a further amplifierstage as represented diagrammatically in Fig. 2 by the gaincharacteristic curve A1. The operating point PA1 of this stage is placedon a linear portion of the curve so that the output wave W4 finallydelivered is again substantially symmetrical and a relatively truereplica of the input wave W. Further amplification may be used and,since each successive stage is out of phase with respect to the next,the additional linear amplification may by proper circuit design, beused to iron out remaining irregularities on both sides of the Waves.

The progress of a signal of lesser intensity through the amplifier isalso shown in dotted lines in Fig. 2 and it will be evident that asignal below the predetermined maximum level will be amplified in usualmanner and without introduction of distortion thus indicating thedesired high weak signal response of the amplifier.

By proper choice of operating potentials the operation of the circuitupon signals above a predetermined level may be selected as desired.Thus the amplifier may be caused to flatten off at the proper level asindicated at D in Fig. 3 or it may be caused to have a rising orfalling-off characteristic as represented at E and F. Actual tests provefurther that the amplifier may even be caused to cut off sharply above aselected level as indicated at G.

The advantages and importance of such operation particularly appear inhearing aid Work since by levelling or cutting off the response above apredetermined maximum the possibility of a signal reaching an intensitysuch as to cause pain to the user is absolutely prevented. The circuithowever, will find uses wherever rap'd and automatic control of gain orvolume may be desirable.

The circuit shown in Fig. 4 is essentially like that of Fig. l employingthree connected vacuum tube amplifiers V1, V2 and V3 with input andoutput I and O, and having screen grid elements I0, and I2 respectively.The circuit has grid resistors I3. interstage coupling condensers I4,plate load resistors I5, IS and I1, screen grid resistor 2| and by-passcondenser 22 for the last tube V3, and common B positive supply I8, allas previously described. In this case, however, the screen grid elementsI0 and I I of tubes V1 and V3 are connected together and through asingle dropping resistor 2`| are connected to the plate of the tube V3,being therefore common to the plate load resistor I'|, and have a singleby-pass condenser 26. I

In this circuit the operation is similar to that heretofore describedand differs only in that the distortion entered in tube V3, and theresulting shift in the direct current signal component, is used tocontrol the voltage and space charge conditions of both screen gridelements I0 and in the tubes V1 and V2. Effective control of the gain ofthese tubes is therefore obtained and both tubes may be, like the firsttube V1 in Fig. 1, set up to normally give linear amplification.

The circuit of Fig, 5 employs the same basic principles of operation butthe distortion entering tube is an "auxiliary tube having no outputconnection such as to enter the distortion into the amplified signal.Here the first two tubes V1 and V2 are arranged to give linearamplification, from the input I, to the output O connected throughcondenser 28 to the plate of V2. Grid biasing resistors I3 and plateload resistors I5 and I6 are provided for V1 and V2 to provideamplification in usual manner. A portion of the output of the secondtube V2 is also taken through condenser 29 to the control grid of theauxiliary amplifier tube V4, which may be a triode and has the gridresistor 30 and a plate load resistor 3| connected to the B positivesupply I8.

The screen grid elements I Il and I| of tubes V1 and V2 are here againconnected together and derive their operating voltage through a droppingresistor 32 connected to the plate of V4 thus making load resistor 3|common to both screen grids. A by-pass condenser 33 for the screen gridsis again used.

In operation the tubes V1 and V2 will amplify and deliver signals to theoutput O, but will also deliver a part of such signals to the tube V1which is so biased and set on its characteristic curve that, above apredetermined level, these signals will cause distortion. The resultingshift in the direct current potential will cause a voltage drop inresistor 3| to reduce and control gain in tubes V1 and V2, the screengrid elements of which are common to said resistor exactly as heretoforespecified. In this case however the distortion entered by the auxiliaryor control tube V4, being mainly of the second harmonic variety, may beby-passed to ground by the condenser 33 and thus does not enter thesignal path through the amplifying tubes V1 and V2 and no furtheramplification, for distortion cancellation at least, is needed.

In Fig, 6 I show a further variation of the circuit of Fig, 5 in whichthe tubes V1 and V2 are connected exactly as described and have the samecircuit elements designated by corresponding reference numerals. A thirdtube V5, also of the screen grid variety, is used but connections are somade that this tube serves two distinct functions. Thus the output oftube V2 is fed through coupling condenser 34 to the control .grid oftube V5 with the usual bias resistor 35 but the screen grid element 3Sof this tube is connected through load resistors 31-38 to the B positivesupply I8 with a decoupling or by-pass condenser 39 at the junction ofthe resistors. The screen grid thus acts as a plate and this portion ofthe tube operates as a. high-mu triode, the output O being taken fromthe screen grid through coupling condenser 4D. Nevertheless the electronflow toward the plate of the tube maintains and this plate is connectedthrough a load resistor 4I to B supply I8. The screen grids I Il and ofV1 and V2 are then connected together, with a by-pass condenser 42, andare connected to the plate of V5 through the dropping resistor 43.

In action a portion of the signal appearing at the erstwhile screen grid36, now acting as a plate of the tube V5, is transferred by the electronflow to the true plate of the tube, due to the electron coupling, and ifsufficient to overload this portion of the tube, will cause distortionand plate current surges in the resistor 4l. The voltage drop in thisresistor will then reduce the voltage on the screen grids of tubes V1and V2 to control the gain thereof according to the signal level. Hereagain the distortion is by-passed by condenser 42 to prevent reaction onthe signal itself and in this circuit the single tube V5 is caused tooperate not only as a signal amplifier but as a distortion entering orcontrol tube.

All of the foregoing circuits have the desirable characteristics ofcontrolling the gain of the first tube or tubes in response to signallevel in later amplifiers and therefore provide control in earlierstages where most needed and in response to the relatively greatamplitudes the signals have after preliminary amplification.

However, as shown in Fig. 7, each tube may be arranged to introducedistortion above a predetermined signal level to control its own gain.In such case the tube, indicated at V6 has the usual input and outputconnections I and O and a plate load resistor 44 of high value, the tubebeing arranged, as heretofore described, to work on a portion of itscharacteristic curve at which distortion will take place above thedesired maximum level. The screen grid 45 is then connected directly tothe plate through dropping resistor 46 and has the by-pass condenser 41.The resistor 44 being thus common to both plate and screen grid willeiiect the screen grid voltage in response to signal induced currentvariations in the plate circuit and thus will control the gain in thesame manner previously described. The action must properly be used intwo or more succeeding stages of amplification to remove distortion fromthe output signal and provide proper control as will be evident.

This circuit is similar in essential respects and in gain controllingaction to those shown in my copending application Serial No. 333,998,for Automatic gain compensating means, filed May 8th, 1940, now PatentNo. 2,282,649, issued May 12, 1942, to which attention is invited forcomparative purposes.

Where signals of minute amplitudes are handled in a circuit such asshown in Fig. 7 the screen grid by-pass may be eliminated, the tube thenacting as a pentode to signals of small amplitude and as a triode tosignals of greater amplitude. Where the screen grid by-pass iseliminated in circuits such as Fig. 1, where control is had from asucceeding tube, then a regenerative-degenerative network must be usedas shown in my application Serial No. 333,998.

It is understood that suitable modifications may be made in thestructure as disclosed, provided such modifications come within thespirit and scope of the appended claims. Having now therefore fullyillustrated and described my invention, what I claim to be new anddesire to protect by Letters Patent is:

1. Gain control means for a vacuum tube signal amplifier, said amplifierhaving at least two tubes and one of said tubes being of the screen gridtype, the other tube being connected and arranged to operate in suchmanner that it will Cal enter distortion on signals above apredetermined level and thereby will cause a large increase in its anodecurrent, said anode current being passed through a load resistor of highvalue, and said load resistor being common with the screen grid of thesaid screen grid tube to supply operating potential thereto and therebyreduce said potential as such increase in anode current occurs, andmeans for by-passing components of such distortion from the screen gridto the common ground of the amplifier.

2. Gain control means for a vacuum tube ampliiier system having at leasttwo screen grid tubes preceding a later amplifier tube, comprising loadresistors arranged to supply operating potentials to the anodes of thetubes, the load resistor of the second screen grid tube being connectedto supply an operating potential to the screen grid of the precedingtube and the load resistor of the said later amplifier tube beingconnected to supply an operating potential to the screen grid of thesecond tube, and said second tube and later amplifier tubes beingarranged to operate in such manner that they will enter distortion orsignals above a predetermined level and cause an increase in anodecurrent fiowing through their load resistors to thereby lower theoperating potential on the said screen grids.

3. In a vacuum tube amplifier system, at least two screen grid amplifiertubes having signal input and output circuits, an auxiliary tubeconnected to receive a part of the output signal and amplify same but toenter distortion when the signal level exceeds a preselected maximum,and means for supplying operating potential to the screen grids of theamplifier tubes under control of the auxiliary tube in such manner thatthis potential will be reduced as the auxiliary tube reaches a conditionof distortion.

4. A multi-stage amplifier system having at least two screen grid tubes,the first of said tubes having signal input means and output to thecontrol grid of the second tube, the screen grid of the second tubebeing supplied with operating potential to act as an anode and having anoutput signal connection, the anode of the second tube being adapted bythe electron flow in the tube to receive a signal voltage and beingoperated in such manner that a condition of distortion and anode currentincrease will occur when the signal level exceeds a preselected value.and means for reducing the operating potential on the screen grid of thefirst tube when such distortion occurs at the anode of the second tube.

5. The method of controlling gain in a signal amplifier employing screengrid tubes in at least a part of its stages, which includes operating atleast one tube under conditions such that distortion will occur onsignals above a predetermined value to thus secure a shift in the steadydirect current component of the signal, and to control space chargeconditions by the screen grids of the screen grid tubes in direct ratioto the distortion and shift in said direct current component.

6. The method of controlling gain in a signal amplifier employing screengrid tubes in at least a part of its stages, which includes operating atleast one tube under conditions such that distortion will occur onsignals above a predetermined valve to thus secure a shift in the directcurrent component of the signal, and to control space charge conditionsby the screen grids of the screen grid tubes in ratio to the distortionand sh'ift in said direct current component, and finally cancelling outsuch distortion.

7. The method of controlling gain in a signal amplifier having at leastone screen grid amplifier tube with input and output connections andanother tube connected to receive signal voltages from said ampliertube, which includes operating said another tube under such conditionsthat it will enter distortion and cause a shift in the direct currentcomponent of the signal when the signal amplitude reaches apredetermined level, controlling the operating potential upon the screengrid of the amplifier tube in accordance with the shift in directcurrent potential to decrease the gain as the signal reaches said level,and by-passing the larger portion of such distortion away from saidscreen grid.

8. In a vacuum tube amplifier system including a screen grid tube, saidtube being adjusted to operate under such conditions as to enterdistortion onsignals above a'preselected amplitude to thereby cause anincrease in its anode current, a resistor arranged to carry said anodecurrent, and the screen grid of the tube being connected to receive itsoperating potential through said resistor whereby said increase in anodecurrent will automatically reduce the potential on the screen grid whenthe signal exceeds said preselected amplitude.

9. In a vacuum tube signal amplifier including a screen grid tube andanother tube with a resistor of high value arranged as its plate load,said another tube being adjusted to operate under conditions such thatit will cause distortion upon signals exceeding a predeterminedamplitude and thereby cause an increase in its plate current drawnthrough said load resistor, and said load resistor being also connectedto supply a variable operating potential to the screen grid of saidscreen grid tube in proportion to the variations in current drawnthrough said load resistor.

10. Gain control means for a vacuum tube ampliiier system having atleast two screen grid tubes preceding a later amplifier tube, comprisingload resistors arranged to supply operating potentials to the anodes ofthe tubes, the load resistor of the said later amplifier tube beingconnected to supply an operating potential to the screen grids of thetwo screen grid tubes, and said later amplifier tube being adjusted tooperate under such conditions that it will enter distortion when thesignal level reaches a predetermined value and cause an increase in itsanode current through said load resistor to thereby reduce the operatingpotential on the screen grids.

11. In a vacuum tube amplifier system including a tube of the screengrid type, said tube being adjusted to operate under conditions suchthat it will enter distortion upon signals of a predetermined amplitudeand thereby cause a shift in the direct current component of the signalin its anode circuit, a resistor arranged to carry said current, and thescreen grid of the tube being connected to receive a variable operatingpotential from said resistor variable under control of anode currentvariations therein.

FRANK J. FALTICO.

